The oxygen-evolving complex (OEC) of Photosystem II (PSII) in green plants, algae and cyanobacteria contains a cluster of four manganese atoms as the catalytic site of photo induced oxidation of water to dioxygen. Manganese K-edge x-ray absorption spectroscopy (XAS) studies have done much to help elucidate the structure of this active site, allowing a working model to be proposed. However, to determine the structure of the calcium cofactor binding site, and clarify its possible proximity to the Mn cluster, another approach is required. This proposal seeks to apply extended x-ray absorption fine structure (EXAFS) experiments at the Ca K-edge to target the calcium cofactor present in PSII membranes from spinach and cyanobacteria. The method outlined is element-specific and should be a more direct, sensitive probe of the Ca binding site in as close to a native, unperturbed state as possible. The structure of the functional calcium binding site will also be probed at various intermediate S-states (S0-S3) of the OEC, allowing us to discern the crucial role of calcium in the mechanism of water oxidation.